LIVERMORE -- A Pleasanton woman was killed and her son critically injured after their car drove off an embankment on westbound Interstate 580 near the Stonecut railroad crossing, according to the California Highway Patrol.

Wanda Pelillo, 57, was riding in the passenger seat of a small red sedan at 3:01 p.m. when the car swerved off the road near Grant Line Road, Officer Kevin Bartlett said. Witnesses said the driver of the vehicle, Pelillo's 22-year-old son, may have been trying to miss a chair in the roadway.

According to the CHP, the car was driving fast enough that it launched 157 feet off the hillside, over the railroad tracks, into the opposite hillside, and rolled down the hill before coming to a stop on its roof.

A big-rig driver who witnessed the crash pulled over before emergency crews arrived and walked down the embankment to find the mother and son trapped inside the car, Bartlett said. Alameda County Fire crews arrived shortly after and pronounced Pelillo dead at the scene before extracting her from the vehicle.

The man was airlifted to John Muir Medical Center, where he remains in critical condition, authorities said. The cause of the crash is under investigation.